FIG. 1 is an illustration of a front-end portion of a color CMOS active pixel sensor (APS) imaging system 100. The system 100 includes a N×M pixel array 110 comprised of pixels R, G, B which are sensitive to red, green, and blue colored light, respectively. The pixels R, G, B are typically arranged in a Bayer pattern, in which alternating rows are comprised of green/red and green/blue pixels in adjacent columns.
Any image focused upon the pixel array 110 causes the pixels R, G, B to convert the incident light into electrical energy. As will be described in greater detail below, this conversion typically results in a pixel outputting two electrical signals, namely the reset signal and a photo signal. Typically, CMOS APS imaging systems 100 operate by selecting and coupling a row of pixels, via signal lines 206 to a sample-and-hold circuit 120. The sample-and-hold circuit 120 includes M subcircuits 121 which are coupled to a bus 122. Each of the subcircuits 121 samples and holds the reset and photo signals from one of the pixels of the selected row. The M subcircuits 121 are sequentially selected to couple (via bus 122, comprising reset signal line 122r and photo signal line 122p) the sampled reset and photo signals to a driver 130. The driver 130 performs additional processing, which may include, for example, signal amplification or conditioning. The output of the driver 130 is then provided to an analog-to-digital converter 140, which converts the sampled signals into a digital signal. The digital signal can then be provided to the remainder (not illustrated) of the APS imaging system 100 for further processing, manipulation, and storage. The above process is then repeated with the next pixel in the selected row by selecting the next sequential subcircuit 121 until the entire selected row has been processed. The above described processing is then performed on another row until every row in the array 110 has been processed. In alternative implementations the rest and photo signals are combined (e.g., subtracted) at the output of the sample-and-hold circuit 121 or in driver 130 so that the analog-to-digital converter receives a combined signal from each pixel.
FIG. 2 is a more detailed illustration of a pixel (R, G, or B). The pixel includes an element, such as photodiode 201, for converting optical energy to electrical energy. The photodiode 201 is sensitive to green light if the pixel is a green (G) pixel, red light if the pixel is a red (R) pixel, or blue light if the pixel is a blue (B) pixel. The pixels are made color sensitive by red, green, or blue filters as well known in the art. The output of the photodiode 201 is coupled to the gate of source-follower transistor 203 via the source/drain terminals of a transfer transistor 202 and node-A. A power source having a potential level of Vdd is coupled to column output line 206 via the source/drain terminals of the source follower transistor 203 and a row select transistor 204, which has its gate coupled via signal line 210 to a ROW signal. The ROW signal is also coupled to other pixels (R, G, or B) in the same row. A power source having a reset voltage potential level of Vrst is coupled to the gate of the source-follower transistor 203 via the source/drain terminals of transistor 205 and node-A. The gate of transistor 205 is coupled to reset signal line 208 to the RST signal. The gate of the transfer transistor 202 is coupled via signal lines 209 and 207 to a transfer (TX) signal.
The pixel (R, G, or B) is operated in a manner to include a two phase process, wherein a reset signal is output during a first phase and a photo signal is output during a second phase. At the beginning of the first phase, the ROW signal of the pixel is asserted, thereby switching on transistor 204. The TX signal is not asserted, thus transistor 202 is switched off, and the signal from the photodiode 201 is isolated from the gate of the source follower transistor 203. The RST signal is asserted, thereby switching on transistor 205 and coupling the Vrst power source to the gate of the source follower transistor 203 via node-A. This causes a current to flow from the Vdd power source to the output line 206. After a predetermined amount of time, the RST signal is then de-asserted, thereby switching off the source follower transistor 203 and terminating the first phase.
The second phase begins with the assertion of the TX signal, which switches on the transfer transistor 202, thereby permitting the signal produced by the photodiode 201 to be applied to the gate of the source-follower transistor 203. This causes a current to flow from the Vdd power source to the output line 206. After a predetermined amount of time, the TX signal is de-asserted, thereby switching off the transfer transistor 202 and terminating the second phase. The ROW signal is then de-asserted, thereby isolating the pixel from the output line 206.
FIG. 3 is an illustration of the sample-and-hold subcircuit 121. The subcircuit 121 includes a portion for sampling and holding the reset signal 301r and a portion for sampling and holding the photo signal 301p. Each of these sections 301r, 301p are coupled in parallel to signal line 301, which is also coupled to the column output line 206 for a column of pixels (FIG. 2) at node-B. Each section 301r, 301p includes an isolation switch 351, 352 coupled in series to a sampling capacitor 302, 303, respectively. Each sampling capacitor 302, 303 is also coupled in series to its own column select switch 355. Additionally, the sample-and-hold subcircuit 121 includes a crowbar switch 353 which may be used to couple the front plates of the sampling capacitors 302, 303. Each portion 301r, 301p further includes a clamping switch 354, located between the sampling capacitor 302, 303 and the column select switch 355, which may be used to couple the back plates of the sampling capacitors 354 to a clamp voltage Vcl. The sample-and-hold subcircuit also includes a switchable current source 356 for the source follower transistor 203 (FIG. 2) of the pixel. The output of the sample-and-hold subcircuit 121 includes a photo signal at node-C and a reset signal at node-D. Nodes-C/D are coupled to respective lines 122p, 122r of a bus 122 (FIG. 1).
The sample-and-hold subcircuit 121 operates as follows. The sample-and-hold subcircuit 121 is initialized by switching on the current source 356 (for line 301), thereby coupling the output line 206 (FIG. 2) of the pixel (R, G, or B) to ground via line 301. Switches 351, 352, and 353 are open. Both switches 354 are closed, and a clamping voltage Vcl is applied to the back plates of capacitors 302, 303. The clamping voltage Vcl is a stable voltage source over time (i.e., it has a very high rejection of variation of supply voltage), and is used to shift the output of the subcircuit 121 (at nodes-C/D) in order to match the output of the subcircuit 121 with the input of subsequent circuitry (e.g., driver 130). Both switches 354 are then opened.
During the first phase of the pixel operation (in pixel R, G, or B of FIG. 2), column select switches 355 are opened. Switch 352 is closed, to couple the reset portion 301r to the pixel output, which charges sampling capacitor 303. Switch 352 is opened when the first phase of pixel output is completed. As the second phase of pixel output begins, switch 351 is closed to permit the second phase pixel output to charge sampling capacitor 302. Switch 351 is opened when the second phase of pixel output is completed. These operations are performed by each of the subcircuits 121 at the same time, since there is one subcircuit 121 provided for each pixel (R, G, or B) in a row. The output of each pixel may be sequentially output by the sample-and-hold circuit 120 by sequentially closing both column select switches 355 in each subcircuit 121, one subcircuit 121 at a time. When both switches 355 are closed in a particular subcircuit 121, it couples the sampling capacitors 302, 303 of that circuit 121 to lines 122p, 122r (FIG. 1) of a bus 122, which couples the sample-and-hold circuit 120 to the driver 130.
The reset and photo signal comprise a differential signal which can be further manipulated by the driver 130 before being digitized by the analog-to-digital converter 140. Alternatively, the photo signal can be subtracted from the reset signal before being digitized. A control circuit 150, which is coupled to the pixel array 110, sample-and-hold circuit 120, driver 130, and analog-to-digital converter 140, may be used to coordinate the operation of these circuits.
One issue associated with APS CMOS imaging systems is that of fixed pattern noise, which is a type of distortion in the image captured by the imaging system. One source of fixed pattern noise is due to imperfections in the sample-and-hold subcircuits 121, including those caused by a failure in a subcircuit to accurately sample the reset and photo signals.
This failure may be due to parasitic capacitance in subcircuit 121. For example, when the subcircuit 121 of FIG. 3 is implemented using conventional fabrication techniques, the result is the subcircuit 121′ illustrated in FIG. 4. The subcircuit 121′ is similar to the subcircuit 121 (FIG. 3), but includes three types of parasitic capacitance. These include a primary cross capacitance 401 which couple the front plates of both sampling capacitors 301, 302, as well as secondary cross capacitances 402, 403, which respectfully couple the front plate of one sampling capacitor 301, 302 to the back plate of the other sampling capacitor 302, 301. The amount of parasitic capacitance may be significant, i.e., ranging approximately five to ten percent of the capacitance of the sampling capacitors 301, 302. For example, when the sample capacitors are 1 pico-farad capacitors, the primary cross capacitance 401 can be approximately 120 femto-farad, while secondary cross capacitances 402, 403 may be approximately 40 femto-farad. The presence of these parasitic capacitances, especially the primary cross capacitance 401, is responsible for a large portion of the fixed pattern noise. Accordingly, there is a need and desire for efficient method for producing a sample-and-hold subcircuit 121 which minimizes the effect of parasitic capacitance.